A device that measures weak biomagnetic signals is very susceptible to the influence of the strong magnetic interferences in its operational environment. This is due to the fact that compared to the biomagnetic signals being measured, the interference signals are even ten million times bigger. Furthermore, the implementation of the interference suppression is made more difficult because the region to be shielded from magnetic interferences is relatively large, tens of centimetres in its diameter.
To make biomagnetic measurements, several methods for protecting measuring devices from interference fields have been developed, which interference fields are many times larger than the interesting signals. A straightforward method of shielding is to place a sensitive magnetic measuring device inside a so-called magnetically shielding room which suppresses magnetic fields originating from sources outside the room into about 100-10,000th part.
In addition to this, to achieve magnetic shielding, it is known to use sensors the geometrical structure of which makes them unsusceptible to rather steady magnetic fields originating from distant sources. Magnetic sensors of this kind are called gradiometers. Typically, a shielding factor of about 100-1,000 against external interferences is obtained using them.
Further, the magnetic shielding can be implemented, or it can be improved, using active systems in which the magnetic interference is eliminated by means of a suitable control system in which the interference is measured in the vicinity of the region being shielded by means of a sensor or sensors; and based on this measurement, the interference field is compensated with current-carrying coils that produce by a magnetic field that is opposing with respect to the interference. Active magnetic shielding can be used either alone or combined with passive shielding methods such as a magnetic shielding room.
In this control system it is possible to use either direct coupling or feedback. When using direct coupling, the measuring device associated with the control system is disposed far from the actuator and from the region being compensated for inside the coil or coils. In this case, the control system functions simply so that one inputs into the coils a current that is proportional to the interference measured by the measuring device and is of the kind that a compensating field having, as accurately as possible, the size of the interference is formed in the region being shielded. With this kind of system it is also possible to combine a magnetic shielding room.
The performance of a compensation based on direct coupling usually is rather limited because the field intensity to be compensated for is determined far from the region being shielded. This functions in the case of one or two stationary interference sources, but when there are three or more sources, it usually is impossible to find a place for the sensor from which the field produced by all the sources could be correctly extrapolated for the region being shielded. This kind of shielding method usually gives a shielding factor of about 3 to 10, depending on the number of interference sources. The method only functions for interference sources that are disposed clearly farther from the region being shielded than the sensor controlling the control system. The method works worse for interference sources that are disposed just a little farther than the sensor controlling the control system, and specifically for sources that are disposed nearer than the sensor it does not work at all.
The sensor of the control system can also be introduced inside the compensation coil assembly near the region in which there is a wish to compensate for an interference. In that case, it is a question about a feedback control system which works better than a directly coupled one also for more complicated interferences originating from many different sources. Publication EP0514027 shows an example of a feedback control system enabling one to minimise the effect of a magnetic interference. With a feedback control system it is also possible to connect a magnetic shielding room either so that the compensation coils are disposed outside the magnetic shielding (U.S. Pat. No. 3,801,877) or inside (EP0396381 or corresponding U.S. Pat. No. 4,963,789).
Publication EP0966689 discloses a magnetic gradiometer which is used to measure diverging components of a magnetic field. In particular, the equipment can be used to measure a small changing field irrespective of the earth's magnetic field (gradient component of the magnetic field). The equipment includes at least two magnetic detectors, feedback coils; and, moreover, each feedback loop includes an amplifier and an integrator. At least two detectors have been adjusted to detect the magnetic field in the same direction. The purpose of the feedback coils is, by imitating the magnetic field produced by the environment, to eliminate the effect of interferences in the total magnetic field to be measured using detectors. To mutually balance the detector outputs, the detector outputs are processed with a signal-processing algorithm. The total energy to be obtained as a sum of the detector outputs is minimised to find out the components of the magnetic field.
In the method as shown in publication EP0966689, a biomagnetic signal being measured becomes distorted as a result of active compensation because the magnetic detectors of the method are used to detect both biomagnetic and interference signals. Publication EP0966689 does not present means to correct this distortion.
In biomagnetic applications, the volume on whose region the sensors are distributed, typically is tens of centimetres in its diameter, that is rather large. If there is a wish, in addition, to keep the set of reference sensors used for the compensation far from the source of interesting biological signals like in the prior art—then the volume containing sensors is even 50 cm in diameter. The compensation of a magnetic interference, e.g. with the accuracy of a percent (its reduction into its hundredth part), by using feedback, requires that the set of compensation coils is capable of producing the fields corresponding to the geometry of the interference fields with the accuracy of a percent in this entire volume that contains both the measurement sensors and the reference sensors that produce the difference signals of the control system. Only in this situation the control system is correctly informed of the interference to be compensated for, and the interference is compensated for in all the measurement loops, with high accuracy.
The compensation using coils producing a compensating field is made the more accurate the smaller is the volume being compensated for. For this reason it would be desirable to place the sensor of a feedback compensation system as near as possible the actual sensors of the measuring device. Previously, one thought that this cannot be done because in that case one would also compensate for the signal to be measured, as if it were an external interference.
The problem with the prior art is thus the inaccuracy of the compensation in the region of the entire set of sensors, because the interference field is measured outside the assembly of sensors. Using a separate set of reference sensors also makes the equipment too complicated.